The present invention relates generally to the management of mobile terminals in a wireless communication network and, more particularly, to a method for using mobile terminal position and previous handoff information to assist in call management.
In a mobile communication network, service areas are typically divided into a plurality of cells, with each cell typically served by a base station. Mobile terminals within a particular cell communicate over RF channels with the base station serving that cell. The base station may handle a plurality of simultaneous calls from mobile terminals. The base stations are connected with one another and to the Public Switched Telephone Network (PSTN) by mobile services switching center (MSCs). The MSCs coordinate the activities of all the base stations and connect the mobile communication network to the PSTN. A typical MSC may handle 100,000 cellular subscribers and 5,000 simultaneous conversations at a time. The MSC also accommodates billing and system maintenance functions. In some higher density networks, several MSCs are used in a single network.
During the course of a call, a mobile terminal may move from one cell into another. A switching technique called a handoff enables the call to proceed uninterrupted when the user moves between cells. When the mobile terminal moves into a different cell while a call is in progress, the MSC automatically transfers the call from the current channel being used to a new channel belonging to the base station serving the new cell.
Handoff decisions are typically based on received radio signal strength and channel quality as monitored by the base station serving the mobile terminal. Received signal strengths are easily monitored and signal quality for any given channel is often determined by the bit error rate or word error rate over a given channel. A handoff from the current channel to another channel is typically initiated when the signal level or channel quality drops below acceptable levels and another channel is available capable of providing acceptable communications. In some systems, a hysteresis algorithm is applied in order to avoid a ping-pong situation in which a second handoff occurs shortly after a first handoff due to the mobile terminal moving slightly back towards the direction of the original base station. To avoid these situations, the candidate cell may be required to be better than the current cell by a given amount, and not just slightly better, for a predetermined is amount of time before a handoff is executed. This results in a delay for those handoffs that are not subject to the ping-pong effect. The use of hysteresis algorithms may result in overall slower handoffs and reduced system capacity
In digital mobile communication networks, such as Global System for Mobile Communications (GSM) and other Time Division Multiple Access (TDMA) networks, it is common for the base stations to enlist the assistance of the mobile terminal to determine when a handoff is required. In order to use a mobile terminal to assist handoff, the serving base station downloads a list of channels, commonly referred to as a neighbor list, at the start of a call or after a handoff. The neighbor list identifies channels in neighboring cells that are potential handoff targets. In a mobile assisted handoff, each mobile terminal measures the received power from surrounding base stations identified by the neighbor list and continually reports the results of these measurements to the serving base station. The signal strength measurements are made by the mobile terminal in-between periods of communication during a call. For example, in GSM and most other TDMA networks, the frequency is divided into eight time slots. The mobile terminal is allocated one time slot for transmissions and another time slot to receive signals from the base station. During the remaining time slots, the mobile terminal is not communicating with the serving base station. The mobile terminal monitors channels belonging to neighboring base stations during these idle periods and then quickly returns to its assigned channels in time to transmit and receive signals in its allocated time slots. The measurements made by the mobile terminal of signals received from neighboring cells are reported back to the serving base station in a scheduled manner so as not to interfere with voice or data transmissions.
The measurement reports provided by the mobile terminal give the base station a list of the signal strength and possibly bit error rates from adjacent cells, as measured by the mobile terminal at its present location. The mobile communication network also knows which adjacent cells have unused radio channels that are available for allocation during a handoff. From the list of available channels, the mobile communication network selects the cell which can best serve the mobile terminal and minimize interference. A suitable traffic channel in that cell is assigned as the target, and the mobile terminal is commanded to retune to the traffic channel in the target cell. At the same time, the call is switched by the MSC from the base station currently serving the mobile terminal to the base station in the target cell. The mobile terminal tunes to the newly assigned channel during one of the idle periods so there is no interruption in transmission. Thus, from the user""s perspective, the handoff can be made seamless. While the discussion of handoff in this paragraph has assumed what is known in the art as a hard handoff, a similar process applies during what is known as a soft handoff, but the mobile terminal may communicate with more than one base station for a short period of time in a soft handoff. As used herein, the term handoff (or hand-off) includes both hard and soft handoffs.
There are resource costs and risks associated with a handoff. Handoffs place significant processing and signaling demands on the mobile communication network, consuming network resources that might otherwise be used to handle other calls. In addition, there is a potential for a short break of the communication when the mobile terminal changes channels. And there is a risk of accidentally losing the call during a handoff. Thus, if the handoff performance can be improved, the overall quality as perceived by end-users will be enhanced.
From the above, it is clear that a significant amount of mobile terminal and network resources and power may be used during handoffs to monitor nearby base stations and otherwise assist the handoff process. Further, it should be clear that, because of the signaling demand a handoff places on the mobile communication network, handoffs should be performed as infrequently as possible and only as needed. Accordingly, there is a need for an improved method for managing handoffs to improve overall performance.
Many calls using mobile terminals are made within cars that are following one or more roads in a given cell and geographic area. Each of these calls should repeatedly experience the same call management and handoff needs because the mobile terminals are traveling the same path as previous users. Currently available mobile communication networks make no use of the fact that a car traveling on a road will most likely be best served by making a handoff at a location that has previously proven successful for mobile terminals that have previously traveled the same road. Therefore, each of these existing networks requires that the calls be handled without taking advantage of previous call experiences.
In one aspect of the present invention, the mobile communication network monitors the position of mobile terminals over time and attempts to match the movement of a given mobile terminal with a predefined route. The current path of the mobile terminal is determined based on the geographic position of the mobile terminal at a number of time instants. This current path is then compared to one or more predetermined routes stored in memory associated with the network to determine whether there is a match between the path and the route. For instance, the comparison may include computing the distance between the current path and the predetermined route over a predefined distance. A metric could also computed that indicates the degree of correlation between the current path and one or more of the routes stored in memory. Information associated with the identified route may then be used to aid in one or more handoffs of the mobile terminal while traveling along the route.
Once a route has been defined and the path of the mobile terminal has been determined to correspond to the route, the network may monitor handoffs occurring along the defined route under other optional aspects of the present invention. When a handoff occurs, the position of the mobile terminal at the time of the handoff is determined and stored in memory. Preferably for each handoff, a quality metric indicating handoff success or quality is determined and stored. The handoff success can be determined based on measurements such as the signal strength, bit error rate (BER), word error rate (WER), or other channel quality measurements before and after the handoff. Separate metrics can be computed for both the uplink and the downlink or a single metric can be computed that takes both uplink and downlink measurements into account. Once a quality metric is determined for a particular handoff position, the quality metric can be used to make future handoff decisions.
In some optional embodiments of the present invention, ping-pong handoffs can also be reduced. Ping-pong handoffs occur when the mobile terminal moves out of and then back into a cell in a short period of time. Routes stored in memory can be used in this situation to avoid multiple handoffs. If the path of the mobile terminal is identified with a route that experiences ping-pong handoffs, the network will delay handing off, assuming signal quality standards are maintained. Thus, the base station in the first cell will continue to service the mobile terminal for a short period while the mobile is within an adjacent cell (from the perspective of the servicing base station). Preferably, the network will monitor the movement of the mobile terminal as it moves within the adjacent cell and will handoff the call when the quality level drops below a predetermined level, the mobile terminal remains within the second geographic area for more than a predetermined time, or deviates from the predetermined, expected route.
Optional embodiments of the present invention also include a method for sending a channel list for the mobile terminal to monitor for mobile assisted handoff (MAHO) management purposes. The network determines the current position of the mobile terminal as is moves through the geographic area. Based on this position, one or more channels are selected by the network for monitoring by the mobile terminal. This enables the mobile terminal to only monitor a limited number of channels providing for more frequent monitoring. Preferably, the network determines the channels to be monitored based on previous handoffs, and will send the channels to the mobile terminal as it nears an expected handoff location.
In still other embodiments, the location and success of handoffs are monitored by the network to aid in identifying areas having poor service quality.
While in some embodiments of the present invention, the predefined routes are supplied to the mobile communication network, some embodiments of the present invention enable the network to learn or define a route from monitoring the geographic positions of mobile terminals as they move through the geographic area serviced by the network. To do so, the network typically monitors the location of the mobile terminals moving within the network and stores the locations. Eventually, the stored locations will reveal a cluster of heavily traveled routes that correspond to roads and highways. The network further monitors the location and quality levels of handoffs. Based upon the historical information previous monitored by the network, a mobile terminal can be identified as moving along a pre-recognized route and appropriate handoff locations can be determined to best maximize the handoff quality.
In other embodiments, the present invention enables the network to learn a route traveled by a single mobile terminal within the network. The network determines the geographic position of the mobile terminal at a number of time instants as it moves within the geographic area serviced by the network, and stores these positions in memory. The network monitors the memory for areas that have a high concentration of positions within a given geographic area and classifies these as routes for that mobile terminal.